D. Bargeman

Zeolite-filled silicone rubber membranes : Part 1. Membrane preparation and pervaporation results

Amongst the alternative fuels obtained from renewable resources alcohol from fermentation may become one of the most important. The combination of fermentation with pervaporation in a membrane bioreactor offers the advantage of continuous processing. In this membrane bioreactor alcohol-selective membranes are needed. The performance of the membranes available at present is poor. Much research is being carried out on silicone rubber but the selectivity of this material for alcohol is too low. Addition to the membrane of a sorptive filler with a high selectivity towards alcohol appears to improve both selectivity and flux. Silicalite, a novel type of hydrophobic zeolite, has been used for that purpose. Results presented in this paper indicate that transport through the zeolite pores contributes to a major extent to the total transport through the membrane.

Stability of Supported Liquid Membranes: State of the Art

Abstract This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed. ∗ Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The Netherlands.

Wetting criteria for the applicability of membrane distillation

Membrane distillation can only be applied on liquid mixtures which do not wet a microporous hydrophobic membrane. Solutions of inorganic material in water have such high values of surface tension (γLgreater-or-equal, slanted72x10−3 N/m) that the non-wetting condition is fulfilled for a number of hydrophobic membranes. As soon as organic solutes are present in the solution, the surface tensionγL will be lowered, and if the concentration of organic material becomes too high, wetting of the membrane will occur. By means of theoretical considerations a critical solute concentration or surface tension at which a homogeneous smooth material will be wetted (gq < 90/deg) can be calculated. For a (micro)porous membranes no such theoretical relation can be derived. Therefore, a simple experimental method is described to measure the maximum allowable concentration for a (micro)porous membrane. On the basis of these measurements, the maximum allowable concentration under process conditions can be determined.

Supported liquid membranes: instability effects

The instability behavior of several supported liquid membranes (SLMs) has been studied for a system in which nitrate ions are removed from an aqueous feed phase and concentrated in a stripping phase. The composition of the aqueous phases and of the membrane liquid has been determined after the aqueous phases had flowed parallel to the membranes for a period of six days. From the experimental data it can be concluded that SLM-failure results from the removal of LM-phase from the support. Contrary to literature data this is not caused by an osmotic pressure difference. It is shown that the membrane stability depends largely on the type of solvent and the molecular structure of the carrier. Furthermore the membrane stability increases with an increasing salt content in the stripping phase (at constant composition of the feed solution).

Transport through zeolite filled polymeric membranes

In this paper the effect of zeolite particles incorporated in rubbery polymers on the pervaporation properties of membranes made from these polymers is discussed. Pervaporation of methanol/toluene mixtures was carried out with membranes prepared from the toluene selective polymer EPDM and the methanol selective polymers Viton and Estane 5707. From the results of the pervaporation experiments it could be concluded that the addition of the hydrophilic zeolite NaX as well as the hydrophobic zeolite silicalite-1 leads to an increase in methanol flux and a decrease in toluene flux through the membranes. Pervaporation experiments with bi-layer membranes consisting of an unfilled polymer layer filled with zeolite particles demonstrated that the effect of addition of particles depends on their position in the membrane. Furthermore, the component flux through the membranes as a function of the volume fraction of zeolite is modelled with existing theories describing the permeability of heterogeneous materials. The results show that the apparent permeability of the dispersed phase is lower than the intrinsic permeability of the dispersed phase when the flux through the particle is restricted by the polymer phase. This phenomenon was confirmed by numerical simulation of the transport in the membrane through a plane parallel to the transport direction. The simulations are carried out for an unfilled membrane, a membrane filled with an impermeable particle, a rubber particle and with a particle which shows Langmuir sorption behaviour. The reason for the discrepancy between the apparent permeability and the intrinsic permeability is that the apparent permeability of the zeolite phase is calculated by dividing the flux with the driving force over the entire membrane which is larger than that over the particle. In case of numerical simulation the concentration in every position in the plane is known and therefore the intrinsic permeability of the filler can be calculated on basis of the actual driving force. This treatment results in a permeability which is correct over several orders of magnitude.

Stabilization of supported liquid membranes by interfacial polymerization top layers

In this paper, a new method of stabilizing supported liquid membranes is presented. The stabilization is based on the application of polymeric top layers to the surface of microfiltration membranes, preventing loss of the liquid membrane phase out of the support pores. The modified microfiltration membranes were used as supports for supported liquid membranes and tested on selective nitrate transport and stability. Screening experiments revealed that most applied top layers did not hinder the transport of nitrate ions. However, a few were able to improve the stability of the liquid membranes. Best results were obtained when piperazine (PIPA) and trimesoyl chloride (TMCl) were used as monomers. For Accurel polypropylene supports with PIPA/TMCl top layer, nitrate flux was constant at the initial 18 × 10−10 mol cm−2 s−1 for 350 h of simulated operation. For uncoated supported liquid membranes (SLMs), the flux decreased within one day from 18 × 10−10 to almost 0 mol cm−2 s−1. Scanning electron microscopy investigations revealed a particular, rippled surface texture of layers prepared with these monomers.

Permporometry: the determination of the size distribution of active pores in UF membranes

Permporometry is a method by which the characteristics of the interconnecting active pores of an ultrafiltration membrane can be measured. It is these active pores that are responsible for the actual membrane performence. Application of permporometry on different membrane types, including ceramic as well as polymeric membranes, shows that the method can provide objective information on the `active? pore size present.

Supported liquid membranes: stabilization by gelation

A new method has been developed to increase the stability of supported liquid membranes. By applying a homogeneous gel network in the pores of the support both the mechanical stability (against liquid displacement) and the long term permeability increase substantially. The flux decreases only slightly because of the open structure of the gel network. A second technique, by which a thin dense gel layer is applied to the feed side of the membrane, results in a specific suppression of the formation of emulsion droplets. The stability of the membrane increases by this treatment to values which are very promising.

Mechanism of supported liquid membrane degradation : emulsion formation

A new hypothesis for the degradation mechanism of supported liquid membranes is advanced: emulsion formation induced by shear forces. Experiments show that the removal of LM-phase from the membrane depends in the molecular structure of the carrier and the type of solvent. The instability of SLMs is regulated by the presence of counter-ions in the same way as in the case of emulsion stability: a decrease in salt concentration in the aqueous phases and an increase in flow velocity of these phases parallel to the membrane surface both lead to an increase in instability of the liquid membrane, while emulsion formation is stimulated by these circumstances.

Zeolite-filled silicone rubber membranes. Experimental determination of concentration profiles

Permeant concentrations in silicalite-filled silicone rubber membranes during pervaporation of propanol/water mixtures were measured using multi-layered membranes. Experimentally determined concentration profiles show that the propanol concentration in the silicalite-filled membrane increases with increasing silicalite content. The water concentration in the membrane is low and no water is present in the silicalite particles during pervaporation. nThe concentration profiles measured here support the observations from the resistance model that the diffusion through the membrane determines the transport rate, i.e., adsorption is a fast process.